The invention relates to a process for recovering and purifying substituted benzenesulfonates, by neutralizing a mixture comprising at least one substituted benzenesulfonate or at least one corresponding substituted benzenesulfonic acid, sulfur trioxide, sulfuric acid and/or hydrogen chloride in free or bound form, in an aqueous phase using an alkali metal hydroxide, and purifying it in this aqueous phase, and by subsequently separating said substituted benzenesulfonate(s) from this aqueous phase.
Substituted benzenesulfonates have a wide range of application. Alkylbenzenesulfonates having more than 5 carbon atoms in the alkyl group are the most commonly used nonsoap surfactants, since they possess good cleaning, emulsifying, foam-forming and wetting properties. Linear alkylbenzenesulfonates possess good biodegradability and are therefore an essential constituent of many laundry detergents and other cleaning agents. Amido acid phenyl ester sulfonates serve as bleach activators in laundry detergents and other cleaning agents which contain bleach. These activators have a number of advantageous properties, such as excellent bleaching performance combined with minimal damage to fabric dyes, good compatibility with washing machines and a good odor profile in laundering.
The prior art discloses many different synthetic routes to substituted benzenesulfonates.
It is known [xe2x80x9cSurfactantsxe2x80x9d in Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A25, p. 747-817, Weinheim 1994, and DE-A 1 131 662] that surface-active alkylbenzenesulfonates can be obtained by sulfonating alkylbenzenes having a side chain containing from 8 to 20 carbon atoms using oleum, removing the excess sulfuric acid and then neutralizing the reaction product using sodium hydroxide. However, the products obtained in this way contain from 10 to 15% of extraneous salts after neutralization, which consist essentially of sodium sulfate, stemming from the excess sulfonating agent, and also from a small amount of sodium chloride, which stems from the bleaching process using sodium hypochlorite that optionally follows the neutralization.
The alkylbenzenesulfonate is freed of these organic salts by known methods involving extraction by organic solvents, such as alcohols or also chlorinated hydrocarbons. However, these processes are laborious and do not always give the required results.
From xe2x80x9cSurfactantsxe2x80x9d in Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A25, p. 747-817, Weinheim 1994, it is further known that sulfonation can be carried out with pure sulfur trioxide, by which means the occurrence of extraneous salts during neutralization is reduced. The sulfonation process using sulfur trioxide in gaseous or liquid form is, however, complicated in view of the apparatus required. Furthermore, the sodium alkylbenzenesulfonates obtained from this process contain a content of extraneous salts (sodium sulfate, sodium chloride) in the percent range based on the active substance.
Furthermore, it is known that chlorosulfonic acid can be used as a sulfonating agent. This method can lead to the partial formation of alkylbenzenesulfonyl chlorides, which lead to the formation of sodium chloride and sodium sulfate as by-products during further processing.
Furthermore, it is known that sodium alkylbenzenesulfonate can be salted out of aqueous solutions, which may also contain dissolved sodium sulfate, using sodium chloride. However, it has been shown that alkylbenzenesulfonates containing low extraneous salt levels are not obtained by this method until after long settling times, and still contain a certain content of sodium sulfate. Therefore, these products cannot be used in the preparation of cosmetic products sensitive to sodium sulfate, or as emulsifiers in industrial scale processes which take place in the emulsion phase.
DE-A 1 131 662 discloses that alkylbenzenesulfonates can be salted out using a mixture which comprises a content of alkylpolysulfonates as well as sodium chloride and sodium sulfate, and that the layer separation between aqueous phase and alkylbenzenesulfonate-rich phase is achieved more quickly as a result, after which the alkylbenzenesulfonates containing low extraneous salt contents are obtained as an aqueous paste and can be separated from the lower layer by known methods. This process has the disadvantage first that the alkylpolysulfonates used end up as undesirable constituents in the product, and secondly that they pollute the wastewater from the process. Also, the settling times required for this process are several hours.
Furthermore, various synthetic routes to amido acid phenyl ester sulfonates are known from the prior art as represented in, for example, WO 95/07882, WO 96/28417, EP 0 922 694 and EP 0 922 695. WO 99/09004 discloses a process for the preparation and purification of amido acid phenyl ester sulfonates.
All the above-described processes for recovery and purification of substituted benzenesulfonates, in particular p-substituted benzenesulfonates, have the problem in common that the substituted benzenesulfonates after operation of the described processes are obtained in consistencies ranging from pastelike to gellike in the presence of water and/or of solvents, so that the substituted benzenesulfonates can only be separated out of the synthesis and/or purification mixtures very slowly and without satisfactory purity. The consequences are uneconomically large separating apparatus and undesirably high product contamination by by-products, such as extraneous salts, or yield losses when washes and/or bleaches are used [xe2x80x9cSurfactantsxe2x80x9d in Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A25, p. 747+817, Weinheim 1994, and DE-A 1 131 662].
It is an object of the present invention to provide a process for the recovery and purification of substituted benzenesulfonates that first does not have the disadvantage of very slow mechanical separation of the product, and secondly achieves satisfactorily high purities.
We have found that this object is achieved by a process for recovering and purifying substituted benzenesulfonates by neutralizing a mixture comprising at least one substituted benzenesulfonate, or at least one corresponding substituted benzenesulfonic acid, and sulfur trioxide, sulfuric acid and/or hydrogen chloride in free or bound form, in an aqueous phase using an alkali metal hydroxide, and purifying it in this aqueous phase, and by subsequently separating said substituted benzenesulfonate(s) from this aqueous phase, wherein said mixture is comminuted into particles having a Sauter mean diameter in the range from 1 xcexcm to 2 cm, either before or during introduction into this aqueous phase.
For the purposes of this invention, xe2x80x9cneutralizationxe2x80x9d and related words such as xe2x80x9cneutralizexe2x80x9d relate quite generally to the neutralization reaction: base+acid=salt+water, and not necessarily to the specific definition of setting to neutral, which is associated with a pH value of about 7.
Particle swarms in most cases do not consist of unitary particles of equal size.
Characterization of the particle size of a particle swarm is carried out in this invention by means of the Sauter mean diameter, since it is decisive in process separating operations using particles subjected to flow, such as filtration. The Sauter mean diameter of a particle swarm is calculated from the volume Vp and the surface area A of all particles of a swarm by the formula: 6Vp/A, see M. Zogg, xe2x80x9cEinfxc3xchrung in die mechanische Verfahrenstechnikxe2x80x9d, pp. 15-17, Stuttgart 1993.